Catalytic and Electrochemical Release of Solar Energy Stored in Strained Organic Compounds

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We envision that solar energy conversion and storage could be integrated via intramolecular reactions in single molecule systems. Such processes would combine key features of photovoltaic and photochemical methods in the most simple and efficient manner. The prototypical example for such an intramolecular reaction is the photochemical conversion of norbornadiene (NBD) to its metastable valence isomer quadricyclane (QC), a single-photon, single-molecule process in which bond-breaking and bond-making events are simplified to the maximum. The reverse reaction from QC to NBD releases up to 100 kJ/mol, making QC a solar fuel with an energy density comparable to state-of-the-art batteries.This project aims at generating the fundamental knowledge to unleash the full potential of chemical energy storage in strained organic molecules. Three major challenges associated with the energy release reaction will be addressed: (1) Its catalytic triggering: Starting from a fundamental understanding of the catalytically triggered cycloreversion and its undesired side-reactions, such as dehydrogenation and C-C bond scission, we will aim at developing catalysts with enhanced selectivity and higher reversibility in the storage cycle. (2) Its electrochemical triggering: Based on a fundamental understanding of the electrochemically triggered cycloreversion, we will aim at improving the reversibility of the storage cycle by design of electrodes and electrochemical environments which limit undesired side-reactions and electrode fouling. (3) The direct conversion to electrical energy: As a grand challenge we envision an "energy-storing solar cell", i.e. the direct conversion of the chemical energy stored in QC to electrical energy. To this aim, we will design hybrid interfaces with appropriate electronic structure, chemical structure and electrochemical stability to demonstrate the feasibility of this new concept.To tackle these very ambitious goals, the groups of Bachmann, Libuda, and Papp have teamed up to combine their complementary expertise in surface science, in-situ spectroscopy, electrochemistry, and materials science. The project team will study the mechanism, kinetics, energetics and stability of the NBD/QC system and its derivatives in ultrahigh vacuum, at ambient pressure, at solid/liquid interfaces and under (photo)electrochemical conditions. Combining aspects of insight gained from single crystal studies, complex model catalysts, and nanostructured materials, we will obtain a detailed understanding of the energy release reaction and the factors that currently limit its reversibility. We will then use this knowledge to develop improved catalytic and electrochemical storage systems, i.e. combinations of molecules, materials, and methods enabling "single-photon, single-molecule" energy conversion and storage in a better controlled, more efficient, and more reversible fashion.